1. Technical Field of the Invention
The present invention relates generally to electro-optical display devices, and in particular to driving electro-optical display devices.
2. Description of Related Art
Traditional active-matrix liquid crystal displays, such as those used in laptop computers, are manufactured by disposing liquid crystal material between a substrate and a glass cover. Individual electro-optical elements defining pixels of an image are created by patterning thin film transistors (TFTs) on the glass cover with a transparent conductive material, commonly indium tin oxide (ITO). To address a particular pixel, the proper row of the matrix is switched on and a charge is sent down the appropriate column of the matrix. A capacitor at the addressed pixel location holds the received charge until the next refresh cycle. However, the fundamental drive signal to set the state of each individual pixel is typically generated externally and provided to the individual pixels through matrix interconnections, which limits the pixel density of active-matrix LCDs.
A more recently developed type of LCD that permits a higher density of pixels than active-matrix LCDs is a liquid crystal on silicon (LCOS) microdisplay. In an LCOS microdisplay, the substrate is an active silicon integrated circuit on which individually controllable electro-optical elements are formed that define pixels of an image. Contained within the silicon substrate is the electronic circuitry used to drive each pixel. Thus, drive signals for the pixels within LCOS microdisplays are generated internally, thereby allowing more pixels per area than active-matrix LCDs. However, the drive voltage in LCOS microdisplays is limited by the breakdown voltage (i.e., the maximum voltage that can be produced and sustained) of the integrated circuit.
Modern integrated circuit processes are utilizing smaller and smaller feature sizes (e.g., 180 nm or smaller), which results in the production of smaller, faster and more power-efficient circuits. Smaller feature size translates into smaller and more densely packed pixels. However, as the feature size becomes smaller, the breakdown voltage decreases. For example, a typical 350 nm complementary metal oxide semiconductor (CMOS) circuit has a breakdown voltage of 3.3V. Smaller electronic components, such as a 180 nm CMOS transistor, typically have a breakdown voltage of only 1.8V.
An important characteristic of LCDs is the display contrast produced by the LCD. The display contrast refers generally to the difference between the optical response of an OFF pixel and the optical response of an ON pixel. To produce the highest possible display contrast, most liquid crystal material manufacturers recommend a drive voltage of 5V. However, when using a CMOS drive circuit containing 350 nm or smaller transistors within an electro-optical display device, such as an LCOS microdisplay, the drive voltage is typically limited to 3.3V or lower, which results in a poor display contrast. Therefore, what is needed is a mechanism for driving an electro-optical display device to increase the display contrast.